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Three new species of the order Monhysterida are described based on specimens obtained at depths of 8081 and 9177 m in the Kermadec Trench. Thelonema clarki sp. nov. is characterised by a large body size (3230–4461 μm), short cylindrical buccal cavity, gubernaculum without apophyses, and long conico-cylindrical tail. This is the first record of the genus since its original description over two decades ago from the Peru Basin. Metasphaerolaimus constrictus sp. nov. is characterised by a relatively long body (1232–1623 μm), slightly arcuate spicules without gubernaculum, and conico-cylindrical tail with inner cuticle conspicuously thickened immediately anterior to cylindrical portion. Monhystrella kermadecensis sp. nov. is characterised by a circle of papillose outer labial sensillae slightly anterior to the four short cephalic setae, gubernaculum with caudal apophyses, the presence of distinct cuticularised piece along anterior vaginal wall, and a relatively short conical (males) or conico-cylindrical tail (females) with conical, ventrally-curved spinneret. M. kermadecensis sp. nov. can be differentiated from all other species of the genus, and, indeed, the entire family, based on the variable position of the anterior gonad relative to the intestine. The new species is classified within the Monhysteridae, and not the closely-related Xyalidae, based on the small body size, a smooth cuticle, and the presence of six outer labial papillae and only one testis. Further work is required to clarify the placement of M. kermadecensis sp. nov. relative to other monhysterid genera. A tabular key to all ten valid Metasphaerolaimus species is presented.
Three new deep-sea species of Thyasiridae (Mollusca: Bivalvia) from the northwestern Pacific Ocean
(2023)
The Thyasiridae is one of the species-richest families in the abyssal and hadal zones of the northwestern Pacific Ocean. Many thyasirid species dominate benthic communities in terms of abundance and play an important role in the functioning of deep-sea ecosystems. Most of the thyasirid species in the region are new to science and have not been described. Based on the material collected from 1954 to 2016 by seven deep-sea expeditions, three new species of Thyasiridae (Parathyasira coani sp. nov., P. pauli sp. nov., and Thyasira kharkovensis sp. nov.) are described from the abyssal and hadal zones (3210–7540 m depth) of the Sea of Okhotsk, the Bering Sea, as well as the Kuril-Kamchatka and Japan trenches. The new species are remarkable among their congeners due to the combination of the following characters: an obliquely-rhomboidal shell with a weak and shallow posterior sulcus and a large prodissoconch with sculpture of lamellated folds. Comparisons with related species are provided.